Invalid_grant with PHPoAuthLib

Hi,

I’m using this library to handle the oAuth:

After authorisation, it redirects back to my application with the code in
the query string but I can’t exchange the code for access token. The error
is always “invalid_grant”.

My POST has the code, redirect_uri, and grant_type. I’ve including my
client id and client secret using Basic auth. I’ve also tried putting them
in the POST body, but nothing seems to work.

Is there any way of getting a (more) helpful error message or is there
anything else I’m missing?

Or can an “official” FreeAgent service
https://github.com/Lusitanian/PHPoAuthLib/tree/master/src/OAuth/OAuth2/Service
for this oAuth library implementation (which is the most popular PHP one on
packagist) be provided?

Many thanks for any assistance!